The order of the JS file in your page and/or master page is important. For example I am using in my MVC project (with date picker) like the next JS files order:
@Content.Script("jquery-1.5.2.min.js", Url)
@Content.Script("jquery.unobtrusive-ajax.min.js", Url)
@Content.Script("jquery-ui-1.8.11.min.js", Url)
@Content.Script("jquery.ui.datepicker-en.js", Url)